It is well known that doorways to public buildings, such as schools, courthouses, etc., present security challenges in preventing unwanted persons from entering through the doors. In a tragic example of a security failure of a common school door, on Dec. 14, 2012, an armed intruder shattered a front door of the Sandy Hook Elementary School in Newtown, Conn., U.S.A. and proceeded to shoot and kill 26 unarmed civilians. The front door of the Sandy Hook Elementary School was only tempered glass that shattered and collapsed upon impact with one bullet fired by the intruder from a gun so that the intruder could then walk through the door into the school.
Many efforts have been made to improve intrusion resistance of glass doors of public and private buildings. For buildings housing high-value persons and or museum-type goods, it is well known to use thick, bullet-proof types of layered glass. For example, U.S. Pat. No. 7,641,965 that issued on Jan. 5, 2010 to Bennison et al. and that is owned by E. I. du Pond de Nemours and Company, shows a multi-layered optically-transparent, ballistic resistant “safety glazing” or window. The Bennison et al. “safety glazing” has an impact zone at one surface, such as an exterior surface of a window of an armored vehicle, and an anti-spall surface to prevent fragments of the window from breaking off and impacting persons within the armored vehicle. In one embodiment, Bennison et al., shows windows having between five and eleven layers. While effective, such a complex window is both prohibitively expensive and too heavy to be utilized in common doorways of public buildings, such as public schools, courthouses, university buildings, hospitals, etc.
More recently, U.S. Pat. No. 7,857,605 that issued on Jan. 11, 2011 to Moran et al. shows an intrusion resistant glass laminate for use between two layers of glass to provide enhanced stiffness without loss of optical clarity. Moran et al. discloses stiffness factors that result in resistance to increasing numbers of axe-head impacts, 19.5 Kg. pendulum impacts, blunt head impacts (for car windshields) and other intrusion risks. Again, while thick, stiff safety glazed, transparent panels provide protection, they generally weigh in excess of 20 pounds per square feet, and hence cannot be efficiently utilized in existing doorways of public buildings.
Consequently, there is a need for a light-weight, shatter-resistant, transparent panel that can be efficiently retrofitted into existing doorways at a minimum cost while providing substantially enhanced resistance to shattering to thereby provide a secure doorway.